1. Field of the Invention
This invention relates to ring laser gyros and, more particularly, to an improved gyro having a reduced sensitivity to bias caused by Langmuir flow and thermal gradients.
2. Description of the Prior Art
Ring laser gyros use two or more counter-propagating light beams of predetermined frequencies to measure the rotational rate about a sensitive axis of the ring as a function of the difference in frequency, i.e., beat frequency, between the counter-propagating beams. The means normally used to produce such counter-propagating beams comprise an electric DC discharge in a gain medium, such as a suitable mixture of helium and neon.
The gas medium will flow during the operation of the ring laser gyro. This flow is caused by two components: an electro-phoretic force directed along the DC excited plasma discharge, causing a Langmuir flow; and a thermal gradient along the gain tube.
The interaction of the light beams with the moving medium gives rise to a frequency shift of the counter-propagating beams. The beam propagating in the direction of the gas flow sees an optical length which is different from that of the beam propagating in the direction opposite to the gas flow. The flow then gives rise to a bias on the beat frequency between the counter-propagating beams. Output information that is gained by rotation of the ring path may thus contain an error due to the bias caused by the plasma flow.
One gyro configuration used to correct the output bias due to the Langmuir flow places a restriction in the passageway of the ring. This configuration is shown in a patent application by Michael Holz titled Ring Laser Gyro, Ser. No. 412,442, filed Aug. 27, 1982, which was filed, as a file wrapper continuation, Ser. No. 948,426, on Dec. 31, 1986. The Holz application places a restriction asymmetrically within the laser passageways.